A high voltage direct current (HVDC) electric power transmission system is a power transmission system that uses direct current for the bulk transmission of electrical power, typically over long distances. An HVDC transmission system that utilizes both underground/undersea cables and overhead transmission lines to transmit electrical power is here referred to as a hybrid HVDC transmission system. Hybrid HVDC transmission systems present a solution to transmit electricity in areas that have environmental constraints or permission issues.
Modern HVDC power transmission systems use electronic converters to make the power conversions that are required for transmission. In particular, a first converter station (commonly referred to as the rectifier station) is used to convert generated AC power into high voltage DC power for transmission over long distances. At the destination end of the transmission system, a second converter station (commonly referred to as the inverter station) is used to convert the received high voltage DC power into AC power that is appropriate for customer use.
One type of HVDC power transmission technology currently in use is known as line-commutated converter (LCC) HVDC power transmission. This type of system uses thyristors to construct the switching elements of the converters and commutation is carried out by the AC system voltage. Another, newer type of HVDC power transmission technology is known as voltage-sourced converter (VSC) HVDC power transmission. This type of system uses IGBTs (or similar power semiconductor devices) to construct the switching elements of the converters and offers certain benefits over LCC technology, such as the ability to control both real and reactive power flow. However, critical issues with VSC HVDC technology are currently preventing widespread deployment of the technology. In particular, all HVDC transmission systems regardless of type require a fault handling system to isolate faults, diminish the fault current, and return the system to full power operation in a short period of time.
A primary issue associated with VSC HVDC systems is that, in the event of a system fault, the energy stored in submodule capacitors of the system will quickly feed the system fault if not appropriately protected against. If a non-local communications channel were to be employed for purposes of implementing a fault protection/handling system, time delays would be introduced, which in turn would impact the speed of protection response. Associated with this communications delay, the capacitive energy has additional time to feed system faults.